Method and system for at least partially updating data encrypted with an all-or-nothing encryption scheme

ABSTRACT

A method for at least partially updating encrypted data stored on one or more servers includes dividing the encrypted data into equal sized chunks; encrypting each chunk using an all-or-nothing encryption scheme (AONE) with an encryption key, wherein an additional randomness per chunk is embedded into the AONE; outputting a plurality of ciphertext blocks for each chunk; storing the encrypted chunks on the one or more servers such that an i-th ciphertext block of each encrypted chunk is stored on an i-th server, wherein a result of a predetermined function performed on the randomness for all encrypted chunks is stored with each encrypted chunk; determining one or more chunks to update; reverting the predetermined function by accessing all the encrypted chunks; decrypting the one or more Chunks to update based on the result of, updating the decrypted chunks; re-encrypting the updated decrypted chunks, and storing the re-encrypted chunks.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application under 35 U.S.C.§371 of International Application No. PCT/EP2014/070986 filed on Sep.30, 2014. The International Application was published in English on Apr.7, 2016 as WO 2016/050287 A1 under PCT Article 21(2).

FIELD

The present invention relates to a method for at least partiallyupdating data encrypted with an All-or-Nothing Encryption scheme storedon one or more servers. The present invention further relates to asystem for at least partially updating data encrypted with anAll-or-Nothing Encryption scheme stored on one or more servers.

BACKGROUND

Although applicable to servers in any kind of network infrastructure,the present invention will be described with regard to a cloud securitynetwork infrastructure.

Cloud security is gaining increasing importance in many applications andservices nowadays. All-or-Nothing Encryption AONE has immerged as one ofthe methods that can be used to strengthen confidentiality of data. TheAll-or-Nothing Encryption AONE provides semantic security of the datawhile ensuring that the data can only be recovered if and only if allthe ciphertext blocks are available for download by a given client. Assuch All-or-Nothing Encryption AONE does not solely rely on the secrecyof the encryption but also requires that any adversary has access to allthe data in order to acquire any meaningful bit of the input plaintext.Therefore, All-or-Nothing Encryption AONE ensures a transparent keymanagement process and complements information dispersal techniques thatcan be used to efficiently store the data in a distributed storage likea cloud.

All-or-Nothing Encryption schemes are for example disclosed in thenon-patent literature of R. Rivest, “All-or-Nothing Encryption and ThePackage Transform” in Proceedings of Fast Software Encryption, pages210-218, 1997, in the non-patent literature of Anand Desai”, TheSecurity of All-Or-Nothing Encryption: Protecting Against Exhaustive KeySearch”, in Proceedings of CRYPTO, 2000 as well as in the non-patentliterature of Ghassan Karame, Claudio Soriente, Krzysztof Lichota,Srdjan Capkun, Technical Report, available from:http://eprint.iacr.org/2014/556.pdf.

Such an All-or-Nothing Encryption AONE may be performed by the followingprocedures. Key generation: On input a security parameter, the keygeneration algorithm outputs a key K. Encryption routine: On inputplaintext p which is comprised of m blocks of size 1 bits each, and oninput an encryption key K, the encryption routine AONEnc outputs n=m+1blocks of ciphertext. Decryption routine. On input key K, and the entireciphertext blocks, the decryption algorithm AONDec outputs the plaintextblocks p. If all ciphertext blocks are not available, then thedecryption algorithm outputs NULL.

An All-or-Nothing Encryption AONE entails that each encryption block ofthe data depends on the data in its entirety, coming at odds withperforming file updates. For example a plaintext file of size of 5 TBcannot be easily updated since it is not workable to simply decrypt andre-encrypt a entire 5 TB every time the new file update needs to beperformed.

A conventional method is for example shown in FIG. 1, relying on theaccess control layer at the server side as a deterrent. However one ofthe drawbacks is that the notion of All-Or-Nothing is relaxed. Indeed,each block output by All-Or-Nothing Encryption only depends on theencryption of m/N other blocks therefore security of the encrypted datais not high.

SUMMARY

In an embodiment, the present invention provides a method for at leastpartially updating encrypted data stored on one or more servers, theencrypted data being encrypted with an all-or-nothing encryption scheme.The method includes a) dividing the encrypted data, which includes afirst number m of plaintext blocks, into a second number N of equalsized chunks, wherein the second number is based on a number of the oneor more servers on which the encrypted data is to be stored, and whereineach chunk includes m/N plaintext blocks, b) encrypting each chunk usingthe all-or-nothing encryption scheme (AONE) with an encryption key,wherein an additional randomness per chunk is embedded into the AONE,and outputting a plurality of ciphertext blocks for each chunk, c)storing the encrypted chunks on the N servers such that an i-thciphertext block of each encrypted chunk is stored on an i-th server,and wherein a result of a predetermined function performed on therandomness for all encrypted chunks is stored with each encrypted chunk,d) determining one or more parts of one or more chunks to update, e)reverting the predetermined function by accessing all the encryptedchunks to acquire the randomness of the one or more chunks to update, f)decrypting the one or more chunks to update based on the result of e),g) updating the decrypted chunks, h) re-encrypting the updated decryptedchunks using the AONE, and i) storing the re-encrypted chunks accordingto step c).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in even greater detail belowbased on the exemplary figures. The invention is not limited to theexemplary embodiments. All features described and/or illustrated hereincan be used alone or combined in different combinations in embodimentsof the invention. The features and advantages of various embodiments ofthe present invention will become apparent by reading the followingdetailed description with reference to the attached drawings whichillustrate the following:

FIG. 1 shows a conventional system;

FIG. 2 shows a system according to a first embodiment of the presentinvention;

FIG. 3 shows a system according to a second embodiment of the presentinvention; and

FIG. 4 shows steps of a method according to a third embodiment of thepresent invention.

DETAILED DESCRIPTION

A method and a system are described herein for at least partiallyupdating data encrypted with an All-or-Nothing Encryption schemesupporting partial file updates on content that has been encrypted byusing an All-or-Nothing Encryption AONE.

A method and a system are described herein for at least partiallyupdating data encrypted with an All-or-Nothing Encryption scheme whichstrengthen the overall security of the encrypted data.

A method and a system are described herein for at least partiallyupdating data encrypted with an All-or-Nothing Encryption schemeenabling an implementation in an easy way and a sufficient performance.

A method for at least partially updating data encrypted with anall-or-nothing encryption scheme stored on one or more servers isdescribed herein that is characterized by:

-   a) Dividing the data comprising a first number of m plaintext blocks    into a second number N of equal sized chunks, wherein the second    number is based on the number of servers on which said data is to be    stored, such that each chunk comprises m/N blocks of the plaintext    blocks,-   b) Encrypting each of the chunks using an All-Or-Nothing Encryption    Scheme with an encryption key, wherein an additional randomness per    chunk is embedded into the All-Or-Nothing Encryption scheme, and    outputting a plurality of ciphertext blocks for each chunk,-   c) Storing the encrypted chunks on the N servers such that the i-th    ciphertext block of each encrypted chunk is stored on the i-th    server, and wherein a result of a predetermined function performed    on said randomness for all encrypted chunks is stored with each    encrypted chunk,-   d) Determining one or more parts of one or more chunks which need to    be updated if any,-   e) Reverting said function by accessing all the chunks to acquire    the randomness of said determined one or more chunks,-   f) Decrypting said determined chunks based on the result of step e),-   g) Updating the decrypted chunks,-   h) Re-encrypting the updated chunks using said All-Or-Nothing    Encryption scheme, and-   i) Storing the re-encrypted chunks according to step c).

A system for at least partially updating data encrypted with anall-or-nothing encryption scheme stored on one or more servers isdescribed herein that is characterized by an updating entity arranged toperform the following steps:

-   a) Dividing the data comprising a first number of m plaintext blocks    into a second number N of equal sized chunks, wherein the second    number is based on the number of servers on which said data is to be    stored, such that each chunk comprises m/N blocks of the plaintext    blocks,-   b) Encrypting each of the chunks using an All-Or-Nothing Encryption    scheme with an encryption key, wherein an additional randomness per    chunk is embedded into the All-Or-Nothing Encryption scheme, and    outputting a plurality of ciphertext blocks for each chunk,-   c) Storing the encrypted chunks on the N servers such that the i-th    ciphertext block of each encrypted chunk is stored on the i-th    serve, and wherein a result of a predetermined function performed on    said randomness for all encrypted chunks is stored with each    encrypted chunk,-   d) Determining one or more parts of one or more chunks which need to    be updated if any-   e) Reverting said function by accessing all the chunks to acquire    the randomness of said determined one or more chunks,-   f) Decrypting said determined chunks based on the result of step e),-   g) Updating the decrypted chunks,-   h) Re-encrypting the updated chunks using said All-Or-Nothing    Encryption scheme, and-   i) Storing the re-encrypted chunks according to step c).

Partial-only file updates are inherently supported by methods andsystems described herein.

A higher level of security compared to conventional security provisionsof conventional All-or-Nothing Encryption schemes is provided by methodsand systems described herein.

Efficiency is enhanced by methods and systems described herein sincefiles which are often accessed, updated, and modified can be more easilyupdated without having to decrypt and re-encrypt the complete fileswithout reducing the level of security.

Resistance against an internal adversary who can steal parts of theinternal memory of the encryption/decryption procedures is provided bymethods and systems described herein.

A simple and easy-to-implement method and system are provided by methodsand systems described herein for updating files.

According to a preferred embodiment, randomness is used as initial seedfor the AONE initialization vector of the All-or-Nothing Encryptionscheme—IVANOE—. This enhances the All-or-Nothing principle, thereforeenhancing the security.

According to a further preferred embodiment the IVAONE is derived fromsaid randomness. This provides a simple and efficient way for providingrespectively deriving the IVAONE.

According to a further preferred embodiment the IVAONE for an (i+1)-thchunk is determined based on performing a hash-function on the IVAONEfor the i-th chunk, wherein the IVAONE for the first chunk is determinedby calculating said hash-function on the randomness. This allows in aneasy and efficient way to derive the IVAONE from the randomness.

According to a further preferred embodiment the randomness is encryptedusing as encryption key an XOR-combination of all ciphertext blocksoutput in step b)—XOREK—and said encrypted randomness is stored in allservers wherein for decrypting according to step e) XOREK is used. Thisensures that entity needs to access all blocks in order to compute theXOR-combination and to decrypt the randomness which is then used todecrypt each underlying All-or-Nothing encryption AONE.

According to a further preferred embodiment a fast-fourier-transformAll-Or-Nothing transform—FFT-AONT—is applied prior or after theencryption using the AONE. Therefore a fast-fourier-transform procedureand an encryption procedure is performed. This ensures that the entitywhich has been revoked needs to store the randomness, the encryption keyand all the blocks of a chunk in order to recover partial the datainstead of only having one access to all blocks: The entity can thenpartially decrypt data if it additionally stores the randomness and theencryption key and additional internal states for example.

According to a further preferred embodiment all data encrypted with theall-or-nothing encryption scheme stored on said one or more servers isdetermined to be updated. This ensures that an adversary who has accessto the data and which has been revoked later on needs the store theencryption key and all the data blocks in order to recover any bit ofinformation.

FIG. 1 shows a conventional system.

In FIG. 1 a conventional system based on access control is providedenabling partial updates of a file.

In FIG. 1 an updating entity UE wants to partially updating a file F.The file F has been encrypted and stored on a plurality of serversSV1-SV4 by the following steps:

In a first step and assuming that they are N servers—here in FIG. 1 N=4servers SV1-SV4—the original plaintext file F is divided into original mplaintext blocks which are in turn divided into N equal sized chunks sothat each chunk contains m/N blocks of the original plaintext blocks andwherein N is the number of servers, here 4.

In a second step an All-or-Nothing encryption AONE is applied on eachseparate chunk C, preferably by using a All-or-Nothing encryption schemeas disclosed in the non-patent literature of Ghassan Karame, ClaudioSoriente, Krzysztof Lichota, Srdjan Capkun, Technical Report, Availablefrom: https://eprintiacr.org/2014/556.pdf. Of course any All-or-NothingEncryption scheme can be used.

In a further step the All-or-Nothing Encryption scheme outputs aplurality of ciphertext blocks for each chunk, denoted by c_(ij),wherein the index i denotes the number of the ciphertext block and theindex j denotes the chunk. For all chunks j=1, . . . , N, the i-thciphertext block c_(ij) is stored in server S_(i).

If it is determined that some part in chunk j needs to be updated onlychunk j is decrypted and re-encrypted using the All-or-NothingEncryption AONE and then stored appropriately on the servers SV1-SV4. Anadversary needs to break into all servers SV1-SV4 and acquire theencryption key in order to be able to decrypt any bit of plaintext. Forexample this helps in resisting against an adversary who can compromiseinternal memory. Using the All-or-Nothing Encryption scheme as e.g.disclosed in the non-patent literature of Ghassan Karame, ClaudioSoriente, Krzysztof Lichota, Srdjan Capkun, Technical Report, Availablefrom: https://eprint.iacr.org/2014/556.pdf the adversary has tocompromise 128N bits of data in order to be able to access anyciphertext of choice later on.

FIG. 2 shows a system according to a first embodiment of the presentinvention.

The method shown in FIG. 1 allows for partial updates of a file butrelaxes the notion of the All-or-Nothing principle. Each output block bythe All-or-Nothing encryption of FIG. 1 only depends on the encryptionof m/N other blocks. In the following a lightweight cryptography basedmethod according to an embodiment of the present invention is shown:

The first step of this embodiment is identical to the first stepdisclosed in FIG. 1.

In a second step the All-or-Nothing encryption AONE is applied on eachseparate chunk C using an encryption key K. In addition to the methoddisclosed in FIG. 1 an additional randomness R is embedded in thecreation of each All-or-Nothing Encryption AONE. The randomness R isused as an initial seed for the initialization vector IV in allAll-or-Nothing Encryption AONE. In the following the initializationvector is not included in the ciphertext but is only derived from therandomness R.

For example the initialization vector for the first chunk may be set toH(R), i.e. a hash function is computed over the randomness R. Theinitialization vector for chunk 2 is H(H(R)) and so on.

In a next step for each chunk the XOR operation of all blocks iscomputed. By denoting Ti the XOR of all blocks in chunk i, therandomness R is encrypted using the new key as an XOR-combination of allTi: T1 XOR T2 XOR . . . XOR Tn. The encrypted randomness is stored onall the servers SV1-SV4. This ensures that an adversary entity needs toaccess all blocks in order to compute T1 XOR T2 XOR . . . XOR Tn and todecrypt the randomness R which is then used to decrypt each underlyingAll-or-Nothing Encryption AONE.

The method shown in FIG. 2 inherently supports partial file updating. Ifsome part of chunk j needs to be updated only chunk j is decrypted inre-encrypted using AONE in stored appropriately on the servers SV1-SV4.

FIG. 3 shows a system according to a second embodiment of the presentinvention.

A method according to the embodiment of FIG. 2 ensures that to require asingle plaintext block any entity has to have access to all ciphertextblocks. However once the entity had one access to all blocks then theentity can partially decrypt data if it additionally stores therandomness R and the encryption key K and additional internal state forexample.

In the following an embodiment is shown ensuring that the entity whichhas been revoked needs to store the randomness R, the encryption key Kand all the blocks of a chunk in order to recover partial data: Ingeneral the embodiment of the method shown in FIG. 3 comprises the samesteps as described in the description of FIG. 2. However in contrast tothe method in FIG. 2 a Fast-Fourier Transform All-or-Nothing Encryptionto each chunk is provided. Also the Fast-Fourier TransformAll-or-Nothing Encryption can be applied to the data in its entirety.This ensures that an adversary having access to the data and revokinglater on needs to store the encryption key and all data blocks in orderto recover any bit of information.

In the following a procedure for performing a Fast-Fourier TransformAll-or-Nothing transform is shown. It is assumed that E(X, Y, Z) is ablock cipher encrypting (Y∥Z) using the key X.

ALGORITHM 1 Algorithm of FTT-AONT. Here, we assume that E(X,Y,Z) is ablock cipher that encrypts Y||Z using key X. for j = 1, . . . , log(n)do  for l = 0, . . . , 2^(j) − 1 do   for i = 0, . . . , 2^(j−1) − 1 do   x^(j)[i+l*(2^(j))], x^(j)[i+l*(2^(j))+2⁽j−1)] = E(K, x^(j−1)   [i+l * (2^(j))], x^(j−1)[i + l * (2^(j)) + 2⁽j − 1)])   end for  endfor end for

FIG. 4 shows steps of a method according to a third embodiment of thepresent invention.

In FIG. 4 in a first step S1 the data comprising a first number of mplaintext blocks is divided into a second number N of equal sizedchunks, wherein the second number is based on the number of servers onwhich said data is to be stored, such that each chunk comprises m/Nblocks of the plaintext blocks.

In a second step S2 each of the chunks is encrypted using anAll-Or-Nothing Encryption Scheme with an encryption key, wherein anadditional randomness per chunk is embedded into the All-Or-NothingEncryption scheme, and a plurality of ciphertext blocks for each chunkis output.

In a third step S3 the encrypted chunks are stored on the N servers suchthat the i-th ciphertext block of each encrypted chunk is stored on thei-th server, and wherein a result of a predetermined function performedon said randomness for all encrypted chunks is stored with eachencrypted chunk.

In a fourth step S4 one or more parts of one or more chunks which needto be updated if any are determined.

In a fifth step S5 said function is reverted by accessing all the chunksto acquire the randomness of said determined one or more chunks.

In a sixth step S6 said determined chunks based on the result of step S5are decrypted.

In a seventh step S7 the decrypted chunks are updated.

In an eighth step S8 the updated chunks using said All-Or-NothingEncryption scheme are re-encrypted, and

In a ninth step S9 the re-encrypted chunks are stored according to stepS3.

The servers SV1-SV4 shown in the FIG. 1-3 can for example be part of amulti-cloud storage system leveraging a number of commodity cloudproviders with the goal of distributing trust across differentadministrative domains. Such a “cloud of clouds” model is receivingincreasing attention nowadays with leading cloud storage providersoffering products for multi-cloud systems.

In FIG. 1-4 a system of 4 storage servers SV1, SV2, SV3, SV4 and acollection of users represented by updating entities UE are assumed aswell as that the servers SV1-SV4 appropriately authenticates the users.

In summary embodiments of the present invention provide the use ofAll-or-Nothing Encryption schemes with a lightweight encryptiontechnique in order to efficiently support partial file updates on datawhich has been encrypted using All-or-Nothing Encryption techniques.

Further, embodiments of the present invention enhance existingAll-or-Nothing encryption schemes by combination of access controlmechanisms and lightweight cryptography to resist against an internaladversary who can steal parts of the internal memory of theencryption/decryption routines.

Embodiments of the present invention have inter alia the followingadvantages: The present invention enables to enhance the security of“hot files.” These “hot files” are often accessed, updated and modified.

Embodiments of the present invention have further the advantage toinherently support partial file updates and still maintain a decentlevel of security when compared to the security provisions ofconventional All-or-Nothing encryption schemes.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive. Itwill be understood that changes and modifications may be made by thoseof ordinary skill within the scope of the following claims. Inparticular, the present invention covers further embodiments with anycombination of features from different embodiments described above andbelow.

The terms used in the claims should be construed to have the broadestreasonable interpretation consistent with the foregoing description. Forexample, the use of the article “a” or “the” in introducing an elementshould not be interpreted as being exclusive of a plurality of elements.Likewise, the recitation of “or” should be interpreted as beinginclusive, such that the recitation of “A or B” is not exclusive of “Aand B,” unless it is clear from the context or the foregoing descriptionthat only one of A and B is intended. Further, the recitation of “atleast one of A, B and C” should be interpreted as one or more of a groupof elements consisting of A, B and C, and should not be interpreted asrequiring at least one of each of the listed elements A, B and C,regardless of whether A, B and C are related as categories or otherwise.Moreover, the recitation of “A, B and/or C” or “at least one of A, B orC” should be interpreted as including any singular entity from thelisted elements, e.g., A, any subset from the listed elements, e.g., Aand B, or the entire list of elements A, B and C.

1. A method for at least partially updating encrypted data stored on oneor more servers, the encrypted data being encrypted with anall-or-nothing encryption scheme, the method comprising: a) dividing theencrypted data, which includes a first number of m of plaintext blocks,into a second number N of equal sized chunks, wherein the second numberis based on a number of the one or more servers on which the encrypteddata is to be stored, and wherein each chunk includes m/N plaintextblocks, b) encrypting each chunk using the all-or-nothing encryptionscheme (AONE) with an encryption key, wherein an additional randomnessper chunk is embedded into the AONE, and outputting a plurality ofciphertext blocks for each chunk, c) storing the encrypted chunks on theone or more servers such that an i-th ciphertext block of each encryptedchunk is stored on an i-th server (SV), and wherein a result of apredetermined function performed on the randomness for all encryptedchunks is stored with each encrypted chunk, d) determining one or moreparts of one or more chunks to update, e) reverting the predeterminedfunction by accessing all the encrypted chunks to acquire the randomnessof the one or more chunks to update, f) decrypting the one or morechunks to update based on the result of e), g) updating the decryptedchunks, h) re-encrypting the updated decrypted chunks using the AONE,and i) storing the re-encrypted chunks according to step c).
 2. Themethod according to claim 1, wherein the randomness is used as aninitial seed for an AONE initialization vector of the AONE.
 3. Themethod according to claim 2, wherein the initialization vector isderived from the randomness.
 4. The method according to claim 3, whereinan initialization vector for an (i+1)-th chunk is determined byperforming a hash-function on an initialization vector for an i-thchunk, and wherein an initialization vector for a first chunk isdetermined by calculating the hash-function on the randomness.
 5. Themethod according to claim 2, wherein the randomness is encrypted usingan XOR-combination of all ciphertext blocks output in step b) as anencryption key and wherein the encrypted randomness is stored on all ofthe one or more servers, and wherein the XOR-combination is used for thedecrypting the one or more chunks to update according to f).
 6. Themethod according to claim 1, wherein a Fast-Fourier-TransformAll-Or-Nothing transform —(FFT-AONT)— is applied prior or after theencrypting each chunk using the AONE.
 7. The method according to claim6, wherein the one or more chunks to update include all data encryptedwith the AONE stored on the one or more servers.
 8. A system for atleast partially updating encrypted data stored on one or more servers,the encrypted data being encrypted with an all-or-nothing encryptionscheme, the system comprising an updating entity configured to: a)divide the encrypted data, which includes a first number m of plaintextblocks, into a second number N of equal sized chunks, wherein the secondnumber is based on a number of the one or more servers on which theencrypted data is to be stored, and wherein each chunk includes m/Nplaintext blocks, b) encrypt each chunk using the all-or-nothingencryption scheme (AONE) with an encryption key, wherein an additionalrandomness per chunk is embedded into the AONE, and outputting aplurality of ciphertext blocks for each chunk c) store the encryptedchunks on the N servers such that an i-th ciphertext block of eachencrypted chunk is stored on an i-th server, and wherein a result of apredetermined function performed on the randomness for all encryptedchunks is stored with each encrypted chunk, d) determine one or moreparts of one or more chunks to update, e) revert the predeterminedfunction by accessing all the encrypted chunks to acquire the randomnessof the one or more chunks to update, f) decrypt the one or more chunksto update based on the result of e), g) update the decrypted chunks, h)re-encrypting the updated decrypted chunks using the AONE, and i) storethe re-encrypted chunks according to step c).